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          <h1 class="post-title" itemprop="name headline">手写bind</h1>
        

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        <center>写个bind</center>
<a id="more"></a>

<p><code>bind</code> 位于 <code>Function.prototype</code> 上，我们来做一个polyfill，边测试变完成手写 <code>bind</code>。
新建目录bind，在其中创建src/index.js和test/index.js。之后操作都在bind目录中进行。</p>
<h2 id="fn-bind-asThis"><a href="#fn-bind-asThis" class="headerlink" title="fn.bind(asThis)"></a>fn.bind(asThis)</h2><ul>
<li>为了获取测试环境，以下代码以<code>bind2</code>作为手写的bind进行测试。</li>
<li>一个函数<code>fn1</code>调用<code>bind</code>方法，这个函数<code>fn1</code>就是<code>bind</code>中的<code>this</code></li>
<li><code>fn1</code>调用<code>bind</code>方法时传入第一个参数<code>{name: &#39;lk&#39;}</code>，返回一个函数<code>fn2</code></li>
<li><code>fn2</code>调用时将这个参数<code>{name: &#39;lk&#39;}</code>作为<code>this</code>调用<code>fn1</code>，返回结果</li>
<li>测试用例test/index.js中，函数<code>fn1</code>直接返回其<code>this</code>，应为调用<code>bind</code>时第一个参数<code>{name: &#39;lk&#39;}</code>，所以断言<code>fn2().name === &#39;lk&#39;</code></li>
<li>执行<code>node test/index.js</code>，无报错，测试通过</li>
</ul>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// src/index.js</span></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">bind</span>(<span class="params">asThis</span>)</span>&#123;</span><br><span class="line">    <span class="comment">//this就是调用bind的函数 此例为fn1</span></span><br><span class="line">    <span class="keyword">const</span> fn = <span class="keyword">this</span></span><br><span class="line">    <span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123; <span class="comment">//返回的函数fn2</span></span><br><span class="line">        <span class="keyword">return</span> fn.call(asThis)</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="built_in">module</span>.exports = bind</span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span>(!<span class="built_in">Function</span>.prototype.bind)&#123;</span><br><span class="line">    <span class="built_in">Function</span>.prototype.bind = bind</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// test/index.js</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> bind = <span class="built_in">require</span>(<span class="string">"../src/index"</span>)</span><br><span class="line"><span class="built_in">Function</span>.prototype.bind2 = bind</span><br><span class="line"><span class="built_in">console</span>.assert(<span class="built_in">Function</span>.prototype.bind2 !== <span class="literal">undefined</span>)</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> fn1 = <span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>&#123;</span><br><span class="line">    <span class="keyword">return</span> <span class="keyword">this</span></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> fn2 = fn1.bind2(&#123;<span class="attr">name</span>: <span class="string">'lk'</span>&#125;)</span><br><span class="line"><span class="built_in">console</span>.assert(fn2().name === <span class="string">'lk'</span>)</span><br></pre></td></tr></table></figure>

<h2 id="fn-bind-asThis-param1-param2"><a href="#fn-bind-asThis-param1-param2" class="headerlink" title="fn.bind(asThis, param1, param2)"></a>fn.bind(asThis, param1, param2)</h2><ul>
<li>完善上一步代码，除了传入第一个参数作为 <code>this</code>，同时传入其他参数。改写代码如下</li>
<li>执行<code>node test/index.js</code>，无报错，测试通过</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// src/index.js</span></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">bind</span>(<span class="params">asThis, ...args</span>)</span>&#123;</span><br><span class="line">    <span class="comment">//this就是调用bind的函数</span></span><br><span class="line">    <span class="keyword">const</span> fn = <span class="keyword">this</span></span><br><span class="line">    <span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">        <span class="keyword">return</span> fn.call(asThis, ...args)</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="built_in">module</span>.exports = bind</span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span>(!<span class="built_in">Function</span>.prototype.bind)&#123;</span><br><span class="line">    <span class="built_in">Function</span>.prototype.bind = bind</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// test/index.js</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> bind = <span class="built_in">require</span>(<span class="string">"../src/index"</span>)</span><br><span class="line"><span class="built_in">Function</span>.prototype.bind2 = bind</span><br><span class="line"><span class="built_in">console</span>.assert(<span class="built_in">Function</span>.prototype.bind2 !== <span class="literal">undefined</span>)</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> fn1 = <span class="function"><span class="keyword">function</span> (<span class="params">p1, p2</span>) </span>&#123;</span><br><span class="line">    <span class="keyword">return</span> [<span class="keyword">this</span>, p1, p2]</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> fn2 = fn1.bind2(&#123;<span class="attr">name</span>: <span class="string">'lk'</span>&#125;, <span class="string">'a'</span>, <span class="string">'b'</span>)</span><br><span class="line"><span class="built_in">console</span>.assert(fn2()[<span class="number">0</span>].name === <span class="string">'lk'</span>)</span><br><span class="line"><span class="built_in">console</span>.assert(fn2()[<span class="number">1</span>] === <span class="string">'a'</span>)</span><br><span class="line"><span class="built_in">console</span>.assert(fn2()[<span class="number">2</span>] === <span class="string">'b'</span>)</span><br></pre></td></tr></table></figure>

<h2 id="fn-bind-asThis-param1-param2-p3-p4"><a href="#fn-bind-asThis-param1-param2-p3-p4" class="headerlink" title="fn.bind(asThis, param1, param2)(p3, p4)"></a>fn.bind(asThis, param1, param2)(p3, p4)</h2><ul>
<li>继续完善，<code>fn1</code> 在调用 <code>bind</code> 时所返回的函数 <code>fn2</code> 在被调用时也传入参数</li>
<li>改写代码如下</li>
<li>执行<code>node test/index.js</code>，无报错，测试通过</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// src/index.js</span></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">bind</span>(<span class="params">asThis, ...args</span>)</span>&#123;</span><br><span class="line">    <span class="comment">//this就是调用bind的函数</span></span><br><span class="line">    <span class="keyword">const</span> fn = <span class="keyword">this</span></span><br><span class="line">    <span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params">...args2</span>)</span>&#123;</span><br><span class="line">        <span class="keyword">return</span> fn.call(asThis, ...args, ...args2)</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="built_in">module</span>.exports = bind</span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span>(!<span class="built_in">Function</span>.prototype.bind)&#123;</span><br><span class="line">    <span class="built_in">Function</span>.prototype.bind = bind</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// test/index.js</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> bind = <span class="built_in">require</span>(<span class="string">"../src/index"</span>)</span><br><span class="line"><span class="built_in">Function</span>.prototype.bind2 = bind</span><br><span class="line"><span class="built_in">console</span>.assert(<span class="built_in">Function</span>.prototype.bind2 !== <span class="literal">undefined</span>)</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> fn1 = <span class="function"><span class="keyword">function</span> (<span class="params">...args</span>) </span>&#123;</span><br><span class="line">    <span class="keyword">return</span> [<span class="keyword">this</span>, ...args]</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> fn2 = fn1.bind2(&#123;<span class="attr">name</span>: <span class="string">'lk'</span>&#125;, <span class="string">'a'</span>, <span class="string">'b'</span>)</span><br><span class="line"><span class="built_in">console</span>.assert(fn2(<span class="string">'c'</span>)[<span class="number">0</span>].name === <span class="string">'lk'</span>)</span><br><span class="line"><span class="built_in">console</span>.assert(fn2(<span class="string">'c'</span>)[<span class="number">1</span>] === <span class="string">'a'</span>)</span><br><span class="line"><span class="built_in">console</span>.assert(fn2(<span class="string">'c'</span>)[<span class="number">2</span>] === <span class="string">'b'</span>)</span><br><span class="line"><span class="built_in">console</span>.assert(fn2(<span class="string">'c'</span>)[<span class="number">3</span>] === <span class="string">'c'</span>)</span><br></pre></td></tr></table></figure>

<h2 id="支持-new-的-bind"><a href="#支持-new-的-bind" class="headerlink" title="支持 new 的 bind"></a>支持 <code>new</code> 的 <code>bind</code></h2><p>使用 <code>new</code> 调用函数时实际进行了以下操作：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">new</span> fn(a)</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> temp = &#123;&#125;  <span class="comment">//1.生成一个临时对象</span></span><br><span class="line">temp.__proto__ = fn.prototype  <span class="comment">//2.在临时对象上改变它的原型链</span></span><br><span class="line">fn.call(temp, a)  <span class="comment">//3.把这个临时对象作为 this 调用 fn</span></span><br><span class="line"><span class="keyword">return</span> <span class="keyword">this</span>  <span class="comment">//4.把 fn 的 this 作为返回值return出来</span></span><br></pre></td></tr></table></figure>

<p>原版的 <code>bind</code> 是支持 <code>new</code> 的</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//在浏览器控制台测试原版 bind</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> fn = <span class="function"><span class="keyword">function</span>(<span class="params">a</span>)</span>&#123;</span><br><span class="line">    <span class="keyword">this</span>.a = a </span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">new</span> fn() <span class="comment">//fn &#123;a: undefined&#125;</span></span><br><span class="line"><span class="keyword">new</span> fn(<span class="string">'lk'</span>) <span class="comment">//fn &#123;a: "lk"&#125;</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> fn2 = fn.bind(<span class="literal">undefined</span>, <span class="string">'lk2'</span>)</span><br><span class="line"><span class="keyword">new</span> fn2() <span class="comment">//fn &#123;a: "lk2"&#125;</span></span><br></pre></td></tr></table></figure>

<p>改写我们的 <code>bind</code> 使其支持 <code>new</code> 调用:</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// src/index.js</span></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">bind</span>(<span class="params">asThis, ...args</span>)</span>&#123;</span><br><span class="line">    <span class="comment">//this就是调用bind的函数</span></span><br><span class="line">    <span class="keyword">const</span> fn = <span class="keyword">this</span></span><br><span class="line">    <span class="function"><span class="keyword">function</span> <span class="title">resultFn</span>(<span class="params">...args2</span>)</span>&#123; <span class="comment">//调用 bind 返回的函数 fn2</span></span><br><span class="line">        <span class="comment">//this //如果使用 new 调用fn2，会生成一个 this ；如果没有使用 new ， 此处的 this 为 global</span></span><br><span class="line">        <span class="comment">//this.__proto__ = resultFn.prototype //这个this的原型链就是 fn2 的原型链</span></span><br><span class="line">        <span class="comment">//以此条件判断是否用 new 调用的 fn2 </span></span><br><span class="line">        <span class="keyword">return</span> fn.call(</span><br><span class="line">            <span class="comment">// this.__prototype === resultFn.prototype ? this : asThis, </span></span><br><span class="line">            <span class="comment">//instanceof 运算符用于检测构造函数的 prototype 属性是否出现在某个实例对象的原型链上</span></span><br><span class="line">            <span class="keyword">this</span> <span class="keyword">instanceof</span> resultFn ? <span class="keyword">this</span> : asThis,</span><br><span class="line">            ...args, </span><br><span class="line">            ...args2</span><br><span class="line">        )</span><br><span class="line">    &#125;</span><br><span class="line">    resultFn.prototype = fn.prototype</span><br><span class="line">    <span class="keyword">return</span> resultFn</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="built_in">module</span>.exports = bind</span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span>(!<span class="built_in">Function</span>.prototype.bind)&#123;</span><br><span class="line">    <span class="built_in">Function</span>.prototype.bind = bind</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// test/index.js</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> bind = <span class="built_in">require</span>(<span class="string">"../src/index"</span>)</span><br><span class="line"><span class="built_in">Function</span>.prototype.bind2 = bind</span><br><span class="line"><span class="built_in">console</span>.assert(<span class="built_in">Function</span>.prototype.bind2 !== <span class="literal">undefined</span>)</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> fn1 = <span class="function"><span class="keyword">function</span> (<span class="params">p1, p2</span>) </span>&#123;</span><br><span class="line">    <span class="keyword">this</span>.p1 = p1</span><br><span class="line">    <span class="keyword">this</span>.p2 = p2</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> fn2 = fn1.bind2(<span class="literal">undefined</span>, <span class="string">'a'</span>, <span class="string">'b'</span>)</span><br><span class="line"><span class="keyword">const</span> object = <span class="keyword">new</span> fn2()</span><br><span class="line"></span><br><span class="line"><span class="built_in">console</span>.assert(object.p1 === <span class="string">'a'</span>)</span><br><span class="line"><span class="built_in">console</span>.assert(object.p2 === <span class="string">'b'</span>)</span><br></pre></td></tr></table></figure>

<h2 id="使用旧的API"><a href="#使用旧的API" class="headerlink" title="使用旧的API"></a>使用旧的API</h2><p>现在手写的 <code>bind2</code> 已经实现了原版 <code>bind</code> 的功能。因为是在做 <code>polyfill</code>，不应该用到所有跟 <code>bind</code> 同时出现的所有新的api，同时必须进行错误处理。使用旧的语法修改代码：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// src/index.js</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> slice = <span class="built_in">Array</span>.prototype.slice</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">bind</span>(<span class="params">asThis</span>)</span>&#123;</span><br><span class="line">    <span class="comment">// this就是调用bind的函数</span></span><br><span class="line">    <span class="keyword">var</span> args = slice.call(<span class="built_in">arguments</span>, <span class="number">1</span>)</span><br><span class="line">    <span class="keyword">var</span> fn = <span class="keyword">this</span></span><br><span class="line">    <span class="keyword">if</span>(<span class="keyword">typeof</span> fn !== <span class="string">'function'</span>)&#123;</span><br><span class="line">        <span class="keyword">throw</span> <span class="keyword">new</span> <span class="built_in">Error</span>(<span class="string">'Function.prototype.bind - '</span> +</span><br><span class="line">        <span class="string">'what is trying to be bound is not callable'</span>)</span><br><span class="line">    &#125;</span><br><span class="line">    <span class="function"><span class="keyword">function</span> <span class="title">resultFn</span>(<span class="params"></span>)</span>&#123;</span><br><span class="line">        <span class="keyword">var</span> args2 = slice.call(<span class="built_in">arguments</span>, <span class="number">0</span>)</span><br><span class="line">        <span class="keyword">return</span> fn.apply(</span><br><span class="line">            resultFn.prototype.isPrototypeOf(<span class="keyword">this</span>) ? <span class="keyword">this</span> : asThis,</span><br><span class="line">            args.concat(args2)</span><br><span class="line">        )</span><br><span class="line">    &#125;</span><br><span class="line">    resultFn.prototype = fn.prototype</span><br><span class="line">    <span class="keyword">return</span> resultFn</span><br><span class="line"></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="built_in">module</span>.exports = bind</span><br><span class="line"></span><br><span class="line"><span class="keyword">if</span>(!<span class="built_in">Function</span>.prototype.bind)&#123;</span><br><span class="line">    <span class="built_in">Function</span>.prototype.bind = bind</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<p><a href="https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Function/bind" target="_blank" rel="noopener">MDN bind</a></p>
<p>完</p>

      
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